A typical MRI technique produces an image of a selected body part of an object under examination by manipulating the magnetic spins of hydrogen atoms or protons in body parts such as fat and water molecules and measuring the signals of the manipulated magnetic spins. The measured responses from the magnetic spins are processed to extract images. A MRI system may be designed to generate different magnetic fields for imaging, including a static magnetic field (B0) along a z-direction to polarize the magnetic spins, gradient fields along mutually orthogonal x, y, or z directions in a xyz coordinate system to spatially select a body part for imaging, and a radiofrequency (RF) magnetic field (B1) to manipulate the spins. MRI techniques may be used to capture the functional changes in body parts or tissues such as the brain perfusion.
In MRI, various characteristic times are generally referred to as T1, T2, T2*, TR, TI, and TE. T1 designates the spin-lattice relaxation time, T2 designates the spin-spin relaxation time, and T2* designates the relaxation time in the transverse plane (with contributions from spin-spin relation, as well as other factors causing de-phasing in the transverse plane). TR designates a repetition time, TI designates an inversion time, and TE designates the echo time.